SPM Fundamentals for the MultiMode
Control Mechanisms and Feedback
Rev. B
MultiMode SPM Instruction Manual
29
an electronic image. The main disadvantage of this method was dif
fi
culty in aligning the contacting
tip’s cantilever and the STM tip directly above it.
Figure 2.2b
Early Contact AFM which allowed Imaging Non-conductive Samples
Preceding the
fi
rst SPMs, some pro
fi
lometers had relied upon optical methods to monitor the rise
and fall of a sharp stylus over sample surfaces. This approach offered good sensitivity by re
fl
ecting
a laser beam off the end of the stylus and into a photodetector to obtain an “optical lever” capable of
detecting even the smallest movements (see
). This approach was then applied to SPMs.
A related method utilized interference to detect shifts in interference fringes.
Figure 2.2c
Optical Lever for Monitoring Tip Movement
Laser beam movement is monitored over two axes: vertically and horizontally. As the tip traces
various surface features, its upward and downward movement shifts the beam between upper and
lower photodiode components, creating voltage differences which are electronically rendered into
height information. Lateral movements of the beam are also monitored, corresponding to frictional
phenomena on the surface.
STM tip
AFM tip
Sample
Flexible cantilever
Scanner
Cantilever and tip
Laser
Photodetector
